1. Field of the Invention
The present invention relates generally to an image forming apparatus, and more particularly to an image forming apparatus which has a plurality of exposing optical systems usable when an image is to be formed from different kinds of documents, or when a synthetic image is to be obtained.
2. Description of the Prior Art
In general, copying machines fall roughly into two types; a first type is one in which an optical image is projected on a photosensitive drum of electrical conductivity previously charged, and the resulting latent image is developed with toner, followed by the transfer of the toner image onto a paper. A second type is one in which an optical image is projected onto a photosensitive sheet coated with photosetting microcapsules containing chromogenic materials to form a latent image by selectively hardened microcapsules. The photosensitive sheet is then pressed thereby rupturing unhardened microcapsules and allowing the chromogenic materials in the capsules to flow therefrom.
When different kinds of originals are copied, for example, one original being a normal paper with prints upon which light is reflected, and other being a slidefilm through which light is transmitted to obtain optical images, respectively, two optical systems are employed to expose the respective originals with the optical axes being partly shared by the two optical systems to guide optical images to the same exposure section. Solutions are proposed; Example (1) is disclosed in Japanese Laid-Open Patent Publication Nos. 58-182629 and 59-198442. They disclose a system under which a Fresnel lens, a screen and the like are placed on an original platen on which light rays transmitted through a slidefilm are projected, and the projected light rays are guided to the exposure section by an optical system housed in the copying machine. Example (2) is disclosed in Japanese Laid-Open Patent Publication No. 54-103023, under which a mirror is switched so as to guide light images from different originals to the same exposure section along the same optical axis.
In general, when light rays transmitted through slidefilms are guided to a photosensitive material so as to form an image, the whole surface of the slidefilm is illuminated by light rays from a light source and a slit is disposed in place on an optical path along which the transmitted light rays are guided to the photosensitive material so as to attain a slit exposure effect at an exposure section of the photosensitive material.
The known copying machines mentioned above are disadvantageous; in Example (1) the Fresnel lens, the screen, etc., have to be mounted on the original table when a slidefilm is copied and they have to be removed when a normal original is copied, thereby involving manual difficulty in switching the copy modes between the normal original and the slidefilm. Furthermore, a space is required to house the Fresnel lens, the screen, etc., when they are not used.
In Example (2), the optical axis along which the light image is directed to the exposure section must be changed by switching the mirror. It is difficult to achieve timely and exact mirror switching and angular adjustment. The optical axis is likely to deviate.
If the slidefilm is subjected to constant exposure to light, it is likely to discolor and/or deteriorate owing to the build-up of heat.